The basic method for fabricating bipolar isoplanar integrated circuits with oxidized isolation is described by Douglas L. Peltzer in U.S. Pat. No. 3,648,125. A thin silicon epitaxial layer, formed on a silicon substrate is subdivided into electrically isolated islands or mesas by a grid of oxidized regions of epitaxial silicon material referred to as oxidized isolation regions or isolation oxide regions. The annular shaped isolation oxide regions defining and separating the epitaxial islands are oxidized through the epitaxial layer to the laterally extending PN junction between the epitaxial layer and substrate, referred to as the isolation junction. The top surfaces of the epitaxial islands and the isolation oxide regions are substantially coplanar.
In the conventional isoplanar process the active and passive integrated circuit elements are then formed in the epitaxial islands by a complex sequence of masking and diffusion steps for selectively introducing P-type and N-type dopant materials into different regions of the epitaxial islands defined by the sequence of masks. In the original isoplanar process all of the bipolar regions of P-type conductivity or N-type conductivity are introduced by diffusion. In a typical diffusion sequence, a uniform oxide layer is grown across the epitaxial layer although a layer of other mask material such as nitride layer may be used. A photoresist layer is spun on the oxide layer, the photoresist layer is exposed through a light mask to a pattern of light, and the exposed or unexposed portions of the photoresist are developed and washed away leaving a photoresist mask pattern. The sequence of steps involved in forming the photoresist mask are sometimes referred to as a dry mask step or dry mask sequence. A dry mask sequence results in an overlying photoresist mask pattern. The overlying photoresist mask pattern is then used as an etching mask for etching the exposed portions of the underlying layer. Typically the portions of the underlying layer exposed through the photoresist mask are selectively removed by a chemical etchant which does not effect the photoresist material. A plasma etch, however, may also be used. The photoresist material is then chemically removed leaving a mask of oxide, nitride or other mask material. The sequence of steps by which an underlying layer is etched through the photoresist mask to produce a mask for introducing dopant material is sometimes referred to as a wet mask step or wet mask sequence because of the typical use of chemical etchants.
A traditional isoplanar process is characterized by the following mask sequences:
1.0 Buried Collector Mask PA1 2.0 Isolation Oxide Mask PA1 3.0 Collector Sink Mask PA1 4.0 Nitride or Self-Aligned Transistor Mask PA1 5.0 Emitter Mask or N.sup.+ Mask PA1 6.0 Base Mask or P.sup.+ Mask PA1 7.0 Contact Mask PA1 8.0 Etc. Metalization Masks
Each of the foregoing mask steps in the sequence with the exception of Base Mask 6.0 involves both a dry mask sequence for forming a photoresist mask pattern, and a wet mask sequence for etching the underlying oxide layer to form an oxide mask. The etching, of course, may be accomplished by either a chemical etch or a plasma etch.
The typical diffusion sequence introduces selective regions of N-type or P-type conductivity material in the epitaxial islands. The alternating dry mask procedure and wet mask procedure result in the oxide mask, or other material mask, which then serves as a diffusion mask for diffusing a dopant material of the desired conductivity type through the openings in the mask layer. Upon completion of the diffusion step, the oxide mask must be stripped away and a new oxide layer grown. The further diffusion steps then follow, each involving both dry mask and wet mask sequences. A complete account of the diffusion steps of the traditional isoplanar process may be found, for example, in the Peltzer U.S. Pat. No. 3,648,125 referred to above.
While Peltzer made a basic contribution to the isoplanar fabrication process by the introduction of isolation oxide regions for isolating the epitaxial islands, a number of disadvantages are attendant upon the continued use of diffusion methods. First, the so called dry mask and wet mask sequences result in exposure of the epitaxial layer to environmental contaminants once the oxide layer has been etched. Furthermore, upon completion of a diffusion step, the oxide layer must be stripped away briefly exposing the entire epitaxial layer to environmental contaminants before the new oxide layer is grown. Second, diffusion affords only imprecise control over introduction of the dopant material because diffusion proceeds laterally away from the mask opening as well as axially into the epitaxial layer. Third, the diffusion sequence of dry masks and wet masks requires an inordinate number of steps which increase the time and expense of the manufacturing process.
As a result, modifications have been introduced by semiconductor manufacturers into the isoplanar process to reduce the number of steps and to increase reliability. For example, some of the diffusion steps have been replaced by implant steps in which the dopant material of the selected conductivity type is introduced into the epitaxial layer by a directed ion beam. The present isoplanar processes for forming active and passive elements in the epitaxial islands may therefore typically comprise a hybrid process of diffusion and implanting steps. Such hybrid isoplanar processes are exemplified by the Farrell et al, U.S. Pat. No. 4,199,380.
The isoplanar process has been modified at Fairchild Camera & Instrument Corporation of Mountain View, Calif., to substitute an emitter implant step for the emitter diffusion step. Thus, the 5.0 emitter mask involves a dry mask step only in which the photoresist mask pattern is formed and used for selectively implanting the emitter regions. An ion beam of, for example, N.sup.+ type dopant material is directed through the selective openings in the photoresist mask for introducing the dopant material into the emitter regions. Similarly, the base regions may be implanted so that the 6.0 base mask step involves a dry mask sequence only, forming a photoresist mask pattern for directing an ion beam, for example, of P.sup.+ type dopant material into the base regions.
The hybrid processes of which applicant is aware generally suffer the disadvantage that at one or more occasions through the fabrication process, the protective insulation layer, be it an oxide layer or a nitride layer must be etched or stripped away exposing the underlying epitaxial layer to environmental contaminants. Furthermore, none of the hybrid processes have achieved a full integration of the implant method for introducing dopant material into the fabrication process while retaining in place a passivating nitride layer as a barrier to environmental contaminants. For example, Wen C. Ko et al in U.S. patent application, Ser. No. 340,395, filed Jan. 18, 1982, now U.S. Pat. No. 4,433,471, entitled "Ion Implanted Memory Cells for High Density Ram", while fully converting to implant methods, do not retain a nitride layer for passivating the structure. While the partial introduction of implant methods increases reliability, there is further opportunity for substantial reduction in the number of process steps and therefore the fabrication time and expense while passivating the structure substantially throughout the process.